West Coast to see weekend rain, mountain snow ahead of cold air blast

A strengthening storm will continue to overspread the western U.S. through the weekend with adverse conditions ranging from heavy rain to feet of snow over the mountains, AccuWeather meteorologists say. The same storm will set the stage for severe weather and blizzard conditions in the nation's midsection to start the work week. Even after the rain and snow comes to a stop, impacts will still be felt in the form of a cold airmass, bringing polar air to much of the Intermountain West.

Already, extreme impacts have been felt from the storm in portions of the Golden State. One especially notable example was on the summit of Ward Peak, located just west of Lake Tahoe and roughly 8,600 feet above sea level, where a weather station observed a wind gust of 165 mph.

While a few inches of rain will fall in some areas along the coast, prompting wet roads and localized ponding, the snow will be the most noteworthy aspect.

"A strong Pacific storm will bring multiple feet of snow once again to the Sierra Nevada, where the seasonable average snowpack is up to 175% of normal in parts of the area," noted AccuWeather Meteorologist Mike Youman.

The substantial amount of snow piling up in the mountains is crucial. Experts say that a greater snowpack means there will be more snow to melt away in the drier months of the spring and summer. The snowmelt can help boost soil conditions as well as water levels on streams, rivers and water reservoirs.

Frequent storms over the past month have helped grow the snowpack substantially in the Sierra Nevada.

Any precipitation is welcome in California where a moderate to exceptional drought is still ongoing according to the United States Drought Monitor. The Sierra Nevada has peaks above 8,000 feet that tend to screen out a lot of moisture from Pacific storms. However, it looks like a fair amount of low-elevation showers and mountain snow will push eastward into the Colorado River basin, where drought and low water levels are also an ongoing concern, forecasters say.


Despite the long-term benefits, travel over Interstate 80 could be nearly impossible during the height of the storm. Snow-packed and slippery roads will be hazardous, and visibility will be sharply reduced due to the intensity of the snow. Several inches of snow could accumulate in just an hour's time.

Before the storm affects California, rain, snow and wind crashed into the Northwest on Friday night, with precipitation increasing in earnest Saturday in much of northern and central portions of California. The precipitation is likely to continue right through the end of the weekend.

"One to 2 inches of rain look likely across the Bay Area from Saturday afternoon through Sunday night," said Youman. "Amounts at the higher end of that range would get San Francisco within roughly a half-inch of the city's rainfall average for the entire month of December, with two-thirds of the month to go," added Youman.

As a large southward dip in the jet stream occurs, Southern California will not avoid the rain from this system. Rainfall will not be as heavy as in the northern parts of the state, but amounts will still be beneficial.

The Los Angeles Basin is expected to receive around 1 inch of rain according to Youman, who noted that this would fall in a period of only 48 hours.

AccuWeather meteorologists say that a few thunderstorms could develop across Southern California this weekend. Thunderstorms this time of year in the region are not unheard of, but they typically need quite a strong storm system to form for them to occur.

"Since the rain can be heavy at times, flooding in low-lying areas will be a concern later through Sunday night," Douty said.

The last soaking rain in the city occurred back on Nov. 7-8 when 0.94 of an inch fell. Rain amounts in San Diego with this upcoming storm are expected reach around an inch. The normal rainfall total for the month of December in the city is 1.67 inches.

On the snowy side of the storm, winter weather will not just be limited to the Sierras and the Pacific coast states. Periods of snow will spread across much of Idaho, Montana and Utah into the weekend, bringing accumulating snow to several cities in the region.

"With no shortage of cold air in place with this storm, accumulating snow will be a possibility at lower elevations, in cities such as Boise and Salt Lake City," AccuWeather Meteorologist Andrew Johnson-Levine explained.

The weather along the West Coast should begin to quiet down Monday as snow moves into the Rockies. The system is then expected to rapidly intensify and become a monstrous storm in the central United States by Tuesday. However, colder air will be quick to arrive behind it.

The chilly weather will be first to arrive in the Northwest. In Seattle, the cooldown will begin Monday night, when low temperatures are held below the freezing mark. Highs on Tuesday and Wednesday will struggle to reach 40 degrees, which is over 5 degrees below average for the time of year.

Farther east, temperatures will turn downright bitter in many locations. For Spokane, Washington, temperatures are forecast to drop into the mid to upper teens by Monday night, with temperatures hovering near 25 degrees. With snow on the ground and a dip in the jet stream in place, these temperatures will be set to remain through much of this week.


"A fresh snowpack will reflect incoming solar energy more easily than bare ground, helping take several degrees off the high and low temperatures," Johnson-Levine explained.

Farther south, where snow won't be as big of an issue, chilly weather is still expected. Phoenix, Arizona, is set to see low temperatures plunge into the 30s Fahrenheit on Tuesday, while San Diego is held in the mid 40s the same night. Both of these temperatures are roughly 5 degrees below average, according to past weather data.

With a dip in the jet stream holding steady, these chilly conditions may be here to stay, lasting through the end of the upcoming week.

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